US3766784A

US3766784A - Pipettor stroke control mechanism

Info

Publication number: US3766784A
Application number: US00153692A
Authority: US
Inventors: C Walker
Original assignee: Sherwood Medical Industries Inc
Current assignee: Sherwood Medical Co
Priority date: 1971-06-16
Filing date: 1971-06-16
Publication date: 1973-10-23
Anticipated expiration: 1990-10-23
Also published as: DE2227731A1; CA954836A; DE2227731B2; GB1389269A; DE2227731C3

Abstract

A pipette assembly for drawing and discharging liquid having a stroke control mechanism that produces a shorter stroke during liquid intake than on liquid discharge to assure complete fluid discharge, the control mechanism including an indexing sleeve carried by a main piston in the pipette, with the sleeve having opposed, axially disposed sets of flexible fingers in annular array that engage cams to positively rotate the indexing member in one direction in step-by-step fashion, bringing high and low stops on the indexing member selectively into engagement with abutments within the pipette.

Description

United States Patent 1 Walker [75] inventor: Clarence L. Walker, Webster Grove, Mo.

[73] Assignee: Sherwood Medical Industries Inc.,

St. Louis, Mo.

[22] Filed: June 16, 1971 [2]] Appl. No.: 153,692

[52] U.S. Cl. 73/425.4 P [51] Int. Cl. B011 3/02, G01n 1/14 [58] Field of Search 128/218 R, 218 F; 222/309; 73/425.4 P, 425.4 R, 425.6

[56] References Cited UNITED STATES PATENTS 3,302,462 2/1967 Pursell 73/4256 3,122,280 2/1964 Goda 222/309 49 62 64 2 SS/E 30 PIPETTOR STROKE CONTROL MECHANISM Oct. 23, 1973 3,506,164 4/1970 Weichselbaum et al 73/425.6

Primary Examiner-Richard C. Queisser Assistant Examiner-Daniel M. Yasich Att0rney-Axel A. Hofgren et a1.

[5 7] ABSTRACT A pipette assembly for drawing and discharging liquid having a stroke control mechanism that produces a shorter stroke during liquid intake than on liquid dis charge to assure complete fluid discharge, the control 17 Claims, 7 Drawing Figures PIPETTOR STROKE CONTROL MECHANISM BACKGROUND OF THE PRESENT INVENTION Pipettes are used in scientific, industrial and medical procedures for the rapid transfer of predetermined volumes of liquid from one receptacle to another. Generally they consist of a barrel having a piston mechanism reciprocable within the barrel so that upon withdrawal of the piston a quantity of liquid is drawn either into the barrel or a disposable reservoir attached to the barrel and upon retraction of the piston assembly the liquid is dispensed from the barrel or reservoir. A problem associated with pipettes is that all of the liquid drawn in on the intake stroke is not dispensed by the piston during the discharge stroke, leaving small amounts of liquid within the pipette barrel which, of course, detracts from the accuracy of any analytical measurements being made using the pipette. This is a particularly difficult problem in those pipettes which draw the liquid into a disposable reservoir, or nozzle tip, rather than directly into the pipette barrel, thereby eliminating cross-contamination on successive operations of the pipette. In this type of pipette, a transfer fluid (air) is used to drive the pipetted liquid into and out of the replaceable reservoir attached to the nozzle of the pipette. The use of an air interface between the liquid and the internal parts of the pipette prevents crosscontamination, but also decreases displacement accuracy due to the compressibility of the air in the system.

There have in the past been attempts to overcome this probelm by providing a shorter intake stroke so that the discharge stroke is longer and assures complete discharge of all the liquid in the pipette. These prior mechanisms generally consist of relatively indexible members that index one or more abutments into engagements with a high stop on one stroke of the piston and a low stop on alternate strokes. These indexing mechanisms have included relatively rotatable camming members that are operative upon axial movement of the piston within the pipette. One of the cam memhers is driven by a manually operable plunger and the other cam member drives the fluidpiston.

There are two primary disadvantages in these prior constructions. One is that the indexing members transfer the entire actuating force from the pushbutton to the piston and thus must be constructed of materials and designed to carry the full actuating loads,thereby complicating the devices and rendering them prone to wear, loss ofaccuracy and failure.

Another disadvantage is that these prior indexing mechanisms .at one or more points during the withdrawal and discharge strokes release the pushbutton momentarily so that the operator loses his feel of the position of the fluid piston and he generally jerks the pushbutton causing errorsin the desired amount or rate of fluid drawn into the pipette or expelled therefrom.

SUMMARY OF THE PRESENT INVENTION.

In accordance with the present invention, a stroke control mechanism is provided for a pipette to produce alternate long and short strokes of the'pipette piston to assure complete fluid discharge from the pipette, with the stroke control mechanism providing smooth and positive indexing of short and long stops into engagement with one or more abutments. The indexing mechanism transfers none of the load from the manual pushbutton actuator to the piston so'that the indexing or stroke control mechanism operates more smoothly and may be less expensively constructed than if load bearmg.

The pipette consists of a barrel having a rigid piston assembly therein with the pushbutton connected to one end of the piston assembly. In this manner there is no lost motion between the pushbutton and the fluid piston thereby permitting more accurate control of the stroke length of the piston, and allowing the stroke control mechanism to be non-load bearing, except for its own frictional resistance.

To accomplish the alternate long and short strokes of the piston, an annular indexing sleeve is provided on the piston having high and low stops that selectively engage abutments within the pipette barrel. To index the indexing member relative to the abutments, opposed sets of flexible axial, cantilevered fingers are provided on the indexing member. Each set of fingers is out of phase with the other set and they engage two cams within the barrel having leading cam surfaces disposedso that the indexing member indexes equal incremental movements on each intakeand each discharge stroke of the piston assembly. It should be understood, of course, that the camming surfaces may be reversed by placing the fingers on the barrel and the cams on the sleeve.

The opposed sets of fingers are flexible and are spaced sufficiently close to one another so that they encapsulate each cam as they ride over it providing positive indexing of the indexible member. Moreover, the opposed sets of fingers aresufficiently close circumferentially with respect to the circumferential length of the cams such that at the end of movement of the piston mechanism in one direction, the-indexing member will be rotated sufficiently so that the lagging opposed finger will be in alignment with its cam surface, thus assuring continued indexing in the same direction upon movement of the piston assembly in the opposite direction.

A fine stroke adjustment is provieed by varying the position of the pushbutton relative to thepiston-assembly,-the pushbutton acting as a stop for one direction of movement for the indexing member which in turn acts as a stop for the piston assembly. Movement in the other direction is arrested by positive engagement of abutting shoulders on the barrel and piston assembly.

This is a much simpler stroke adjustment mechanism i than provided in prior devices. An advantage in the opposed finger relationship is that it always maintains the correct timing with the cam and thus there is no requirement for any part to be rotationally locked to the pipette barrel asin. prior devices. A minimum amount of work is required to index the stroke change mechanism since only one part changes position and the low force required tobend the flexible fingers minimizes the load added to the piston return spring. This minimal work reduces force changes and eliminates mechanism impacts during stroking of the piston mechanism. Thus the action of the stroke control mechanism is not discernible by feel to the operator. This allows full control of the stroke rate during the entire expulsion stroke, thereby providing maximum repeatability of successive pipetted volumes. Moreover, the minimal total work means the operators hand will be less fatigued and he can maintain accurate control over a longer period of the working day. Another advantage in the present construction is that the reduced forces in the stroke control mechanism minimize wear and the possibility of stress failure. One reason for this is that the camming surfaces for rotating the indexing member are not the stopping surfaces. Another reason is that the stopping surfaces are flat and extend approximately 166 of the l80 available for each stroke. The stopping construction employs the direct compression of a solid flange on the indexing member which is advantageous since most materials are strongest in compression.

Due to the location of the indexing sleeve adjacent and in engagement with the pushbutton, the forward stopping forces are contained entirely in the pushbutton and the abutments within the housing without any stress being placed on the plunger or the fine stroke adjusting mechanism.

The ideal force path provided by the stroke control mechanism, the zero impact forces effected thereby, the low stress and wear therein, all contribute to maintaining the accuracy of the pipetting device over a much longer period of time.

In another embodiment of the present invention, an axially movable sleeve is provided in the barrel carrying the cams and the abutments for theindexing member. This permits the indexing member and the cam and abutment sleeve to be used in a plurality of different pipettes having widely different stroke lengths, thereby standardizing parts and reducing mold costs.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal section of a pipette assembly according to the present invention;

FIG. 2 is a fragmentary view of the pipettebarrel with the cam and abutment sleeve inserted therein;

FIG. 3 is an enlarged fragmentary view of a portion of the indexing mechanism cooperating with the cam and abutment sleeve;

FIG. 4 is an unrolled schematic view of the indexing member in solid lines and the cam and abutment sleeve in dotted lines; v

FIGS. 5 and 6 are unrolled views similar to FIG. 4 with the indexing member rotated 45' further in each view; and

FIG. 7 is a fragmentary section of a pipette assembly according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referringto the drawings, and particularly FIG. 1, a pipette assembly 10 is illustrated consisting generally of a cylindrical barrel l2 slidably receiving a piston mechanism l4 actuated by a manual pushbutton 16 at the upper end thereof projecting from the barrel l2 and a disposable reservoir or nozzle tip ISattached to the lower end 20 of the barrel 12. The operator depresses the pushbutton l6, inserts nozzle tip into a reservoir,

7 and thereafter releases pushbutton l6 drawing liquid a threaded end 32 that is received in a bore 34 in plastic pushbutton 16'. The bore 34 is unthreadcd prior to force threading it over the threaded end 32. Flange 26 engages annular shoulder 37 in the barrel I2 defining the upper limit of travel of the stepped plunger 24 and piston assembly 14.

The piston or plunger portion 25 is slidably received in cylinder 39 barrel portion 41 defining a chamber 44. As piston 25 moves upwardly in the cylinder, air is drawn into chamber 44 through

axial passages

46 and 47 and liquid is drawn into reservoir tip 15.

A piston assembly return spring 48 is provided in the upper end of the barrel l2, biased against the lower surface 49 of the pushbutton 16 urging flange 26 on plunger 24 into engagement with the shoulder 37, so that upon release of the pushbutton 16 the piston assembly 14 will always move to its upper position with piston 25 withdrawn in cylinder 39.

The indexing mechanism 22 provides alternate short and long strokes for the piston assembly 14. That is, the piston assembly 14 always retracts to the same position determined by shoulder 37 in engagement with flange 26. Upon initial depression of pushbutton 16 the indexing mechanism 22 permits the piston assembly 14 to move a predetermined distance. After release of the pushbutton l6 and upon subsequent depression the indexingmechanism 22 stops the piston assembly 14 at a different position. Thus, the stroke control mechanism 22 effects alternate short and long strokes of the piston mechanism 14, which may, for example, differ by approximately one-eighth of an inch. In this manner discharge from chamber 44 of an excess of fluid over that taken in onthe previous stroke is assured.

The indexing mechanism 22 includes a cylindrical cam sleeve 51 seated on the barrel shoulder 37. The cam sleeve 51 has a flange 52 serving as a spring retainer for spring 48. At the upper end of the cam sleeve 51, disposed in the interior thereof, are two cams 53 disposed 180 apart within the cam sleeve 51. The earns 53 have

camming surfaces

54, 55, 56 and 57 with a truncated surface 59 to permit clearance for other parts as will appear'more clearly below. Surfaces and 57 are parallel as are

surfaces

54 and 56. The cams 53 cooperate with an indexing member 60 to cause relative rotation therebetween as will appear more clearly hereinbelow. Two projections 62, 63 (see FIGS. 2 and 4) project upwardly from the upper end of cam sleeve 51 adjacent the cams 53 and serve as abutments for stops on the indexing member 60.

The indexing member 60 is a plastic molding and includes a tubular portion 64 freely rotatablymounted on reduced shaft portion 30 of plunger 24'. Extending axially downward from the upper end of the .member 60 are four fingers'or, beams 66 having chamferred end surfaces 67 and 68. As seen in FIG. 6, these fingers 66 are cantilevered and are flexible in a circumferential direction. Another set of fingers 69 project upwardly from the lower end of member 60 and are of the same shape and flexure as the-upper fingers 66. There are four

lower fingers

69, 69a, 69b, 69c and they are 45 out of phase with respect to the four

fingers

66a,

66b

66c, 66d, asseen clearly in FIGS. 4-6. The alternate pairs of

fingers

66 and 69 engage the cams 53 to cause relative rotation between indexing member 60 and cam sleeve 51 to index stop surfaces 70 and 71 on the upper end of member 60 into engagement selectively with

abutments

62, 63 on cam sleeve 51. When one

abutmerit

62 or 63 enages a stop 71, a short stroke of plunger 14 is provided, while when one

abutment

62 or 63 engages a stop 70 a longer stroke of piston assembly 14 is effected.

The stops 70 and 71 are formed on an annular ring portion 73 integral with the top end portion of indexing member 60. The annular ring portion 73 extends radially outward from the fingers 66 so that the fingers may engage the earns 53 at the same time the

stops

70 or 71 engage the

abutments

62, 63, as best shown in FIG. 3.

As seen in FIG. 4, the

fingers

66 and 69 are spaced circumferentially a distance substantially equal to the circumferential length of the

cam portions

55, 56, and are spaced axially a distance less than the axial length of one of the cam surfaces 55, 56. The distance between the chamfers 68 on adjacent fingers 66 and the chamfers 75 on adjacent fingers 69 is less than the distance between the cam surfaces 54 and 56 so that the

appropriate fingers

66 and 69 must spread slightly or bend as they ride over the cam surfaces 54 and 56 simultaneously. In the position shown in FIG. 4, fingers 66 have passed over point 77 on the cams and the fingers have returned to their unflexed positions.

As the indexing member 60 is moved downwardly when button 16 is initially pressed,

fingers

66a and 66b ride down

cam surface

55a and 55b, rotating indexing member 60 to the right from the position shown in FIG. 4, to almost the position shown in FIG. 5, aligning stops 71a and 71b with abutments 62, 63and providing a short stroke for piston 25. With the button 16 fully depressed stops 71 will engage

abutments

62, 63 and

fingers

69a and 69b will snap from the dotted line position of FIG. 5 to the solid line position. Upon subsequent release of button 16,

fingers

69a and 69b ride up cam surfaces 56a and 56b, rotating indexing member 60 another 45. Upon the next depression of pushbutton 16,

fingers

66c and 66d have snapped over the tip of cams 53 and ride down cam surfaces 55a and 55b aligning the

stops

70a and 70b with

abutments

63 and 62, respectively, producing a long stroke for piston 25.

The stroke volume may be adjusted by varying the position of pushbutton 16 with respect to the plunger portion 32. Toward this end, the indexing member 60 is seated loosely between the bottom surface 49 of button l6 and shoulder 81 on guide portion 28 ofplunger 24. Upper surface83 of the indexing member 60 is forced into engagement with the lower surface49 of the pushbutton 16 when the

stops

70 or 71 engage

abutments

62 and 63. Thus, by changing the position of surface 49 with respect to the plunger 24 the lower limit of travel of piston 25 may be varied within certain limits. This, of course, does not affect thedifference in stroke length betweenthe intake stroke and the delivery stroke as determined by the stroke control mechanism 22. The stroke control differential is controlled by the difference in height between

stops

70 and 71.

i To prevent the button 16 from clamping the indexing member against shoulder 81, a shoulder 85 is provided near the upper end of shaft portion that engages a counterbore 86 in the button 16 preventing this unwanted clamping. The axial distance between shoulder 85 and shoulder 81 is greater than the sum of the axial length of member 60 and the axial depth of the bore 86.

While the operation of the device is believed clear from the above description, the following discussion will serve as a summary of the invention.

Prior to drawing liquid into the pipette 10, a disposable nozzle tip 15 is telescoped over tip 20. The indexing member 60 is in its upper position shown in FIGS. 1 and 4. Upon depression of pushbutton l6,

fingers

66a and 66b ride down cam surfaces a and 55b which will start rotating the indexing member to the right as shown in FIG. 4. As the

fingers

69a and 69b pass over the corners 90, both sets of

fingers

66 and 69 will spread apart slightly and the

fingers

69a and 69b will begin riding short cam surfaces 570 and 57b. The flexing of

fingers

66a and 66b and 69a and 69b under these conditions assures the positive rotation of indexing member 60 in only one direction.

As the

fingers

66a and 66b reach the end of the cam surfaces 55a and 55b with continued depression of button 16, the ends of the

fingers

69a and 69b will snap over the cam points 91 and 92, respectively, and move to the solid line position illustrated in FIG. 5. This assures that the

fingers

69a and 69b will be on the leading

cam surfaces

56a and 56b on the short retract stroke.

During the continued downward depression of button 16, the sides of the

fingers

66a and 66b merely ride axially in engagement with cam points 94 and 95 without any rotation of the indexing member 60, until the stops 71a and 71b engage the

abutments

63 and 62, respectively. This arrests movement of the plunger assembly at its short stroke position.

The pipeete is then inserted in the reservoir of fluid and the pushbutton 16 is released slowly, drawing fluid into the reservoir of nozzle tip 15 This drives the indexing member 60 upwardly without rotation for a short distance until the indexing member 60 assumes its position with respect to the cams 53 shown in FIG. 5. In this position the

fingers

69a and 69b engage

cam surfaces

56a and 56b and the resulting camrning action begins to rotate the indexing member 60 on shaft portion 30 again to the right. As the

fingers

69a and 69b reach the end of their cam surfaces, the fingers 66c and'66d are released by cam corners 77 so that they are aligned with the beginning of

cam surfaces

55a and 55b. The continued upward movement of the plunger 14 and the index member 60 occurs without rotation of the index member with the

fingers

69a and 69b merely riding axially along

cam corners

94 and 95,. respectively.

After further upward movement of the plunger, flange 26 engages shoulder 37 limiting further upward movement of the plunger and completing the filling of the pipette. Upon discharge, with plunger assembly 14 moving downwardly, the indexing member 60 moves axially a short distance without rotational movement until

fingers

66c and 66d engage

camsurfaces

55a and 55b (as shown in FIG. 6) with the resulting interaction again causing rightindexingrotation of the member 60. Rotational movement of the indexing ring'60. continues until the stops a and 70b are aligned with and engage the

abutments

63 and 62, respectively. Since

stops

70a and 70b are above stops 71a and 7111,11: plunger. is permitted to move an incremental axial distance further on the fluid discharge stroke, thereby assuring complete fluid discharge from the nozzletip 15. The spaces or

slots

97 and 98 between these abutments are, as shown, deep enough to permit this engagement.

In FIG. 7 a somewhat modified form of the present invention is shown, differing basically from the construction shown in FIGS. 1 through 3 in that the cam sleeve is axially movable.

Pipette 110 is seen to include a barrel 112 having a plunger assembly 114 slidable therein and biased to its upper position by a spring 116. The upper position of the piston assembly 114 is determined by flange 118 when in engagement with shoulder 120 on the inside of the barrel. The upper end of the stepped piston 114 carries a pushbutton 119 for actuating the same.

Rotatable on reduced piston portion 121 is indexing member 124 similar to indexing member 51 in the embodiment shown in FIGS. 1 to 3.

Slidable on the piston portion125 is a cam sleeve 127 which has cams 130 and abutments 131 in the same manner as in FIGS. 1 to 3 embodiment.

The difference in the FIG. 7 embodiment is that the cam sleeve 127 is axially movable within the barrel 112. Moreover, the cam sleeve 127 is freely rotatable within the barrel 112. Otherwise, indexing is accomplished in the same manner as in the embodiment shown in FIGS. 1-3. Indexing in this embodiment is effected by the relative movement between the indexing member 124 and the cam sleeve 127. Indexing member 124 has two sets of opposed fingers which are identical to the

fingers

66 and 69 of the embodiment of FIG. 1, but only some fingers of one set, indicated at 128, and some fingers 128a of the other set appear in FIG. 7 because the sets of fingers are angularly offset from each other. Also, the cam sleeve 127 is shown similar to cam sleeve 51 of FIGS. 1 and 3 and has two cams 130 which are identical to cams 53 (FIG. 2). Thus, the two sets of fingers, including

sets

128 and 128a, and the cams 130 are identical to the corresponding parts in the embodiment of FIG. 1. While the cam member 127 is axially movable and rotatable within barrel 112, the forces and friction between members effects relative motion between the indexing member and the sleeve 124 to effect relative movementbetween them for indexing.

On axial downward movement of the button 119 the depending fingers 128 associated with the indexing member 124 engage the cams 130 driving the cam sleeve 127 downwardly until it engages the abutment defined by shoulder 120. The cam sleeve and indexing member index relative to one another bringing one set of the stops on the indexing member into engagement with the abutments 131; Y I

By releasing the button 119 the indexing member moves axially upward, indexing with respect to the cam sleeve 127. The indexing geometry described with re-' spect to FIGS. 4, 5 and 6 is the same so that a repetition is not'necessary'. I

One of the advantages of the FIG. 7 embodiment is that'the indexing member 124 and cam sleeve 127may be used in, a variety of different-pipettes having different stroke lengths. The only difference between pipettesneed be the location of shoulder 120 in order to effect a change in stroke length. There is, thus, an interchangeabiiity of parts which results in a considerable cost saving with this construction.

l claim: 1. A pipette assembly, comprising; barrel means, reciprocal piston means slidable in said barrel means to draw fluid into and expel fluid from areservoir communicating withsaid barrel means, pushbutton means for actuating said piston means, means providing alternate changes in stroke length including an indexing memberrotatably carried by said piston means, said indexing member having high and low stops thereon, abutment means for selectively engaging said high and low stops on the indexing member, means selectively rotating the stops into alignment with the abutment means including a plurality of axially opposed fingers on said indexing member, cam means engageable with said fingers to rotate the indexing member, said cam means including a first cam surface engageable with one set of opposed fingers and a second cam surface engageable with another set of fingers, said fingers being circumferentially spaced so that when one of the fingers of one set completes engagement with said one cam'surface, one of the fingers of the other set is in axial alignment with a portion of the other cam surface so that the indexing member continues rotation in the same direction upon reverse movement of the piston means.

2. A pipette assembly as defined in claim 1, wherein at least one set of said fingers is flexible, the fingers of one set being out of phase rotationally with respect to the fingers on the other set, the fingers being sufficiently long so that the distance between opposed fingers is less than the distance between the cam surfaces thereby requiring bending of the flexible fingers and providing positive rotation of the indexing member.

3. A pipette assembly as defined in claim 2, including four fingers in each set cantilevered on said indexing member, said fingers each being chamferred to provide camming surfaces parallel to the cam surfaces of said cam means, said cam having four camming surfaces, each of said surfaces being parallel to another of said surfaces thereby defining a parallelogram.

4. A pipette assembly as defined in claim 2, wherein said indexing member is a sleeve surrounding said piston means and rotatable relative thereto, each set of fingers consisting of four fingers cantilevered at each end of the sleeve and projecting toward the other set, orieset offingers Benign? ofit ofphase with the other set of fingers.

5. A pipette assembly as defined in claim 2, said stops on the indexing member including an annular flange on one end of the indexing member, and recessed portions on said flange defining alternate highand low stops.

6. A pipette assembly as defined in claim 6, wherein said abutment means is stationary with respect to the barrel means. I a 1 7. A pipette assembly comprising barrel meansfor communication with a fluid reservoir, piston means axially movable in said barrel means to draw fluid into and expel fluid from the reservoir respectively on-intake and discharge strokes of said piston means, means in cluding pushbutton means for actuating said. piston means to move said piston means in oneand, the opposite direction, and means controlling changes in the stroke length of the piston means and operatively connected with said piston means and said pushbutton means, said means controlling changes in the stroke. length comprising an indexing member having con nected thereto opposed first and second sets of circumferentially spaced fingers, and a cam member having at least one cam means thereon engageable with said fingers, one of said members being axially movable relative to the other of said members in response to axlal movement of said piston means in one direction to effect camming engagement between said cam means and at least one of said fingers of said first set, saidcam means and said one finger of said first set effecting relative indexing rotation of one of said members in a prein response to axial movement of said piston means in the opposite direction to effect camming engagement between said cam means and one of said fingers of said second set, said cam means and said one finger of said second set effecting relative indexing rotation of said rotated one of said members in said predetermined direction of rotation by said last named camming engagement, first engagement means on said axially movable one of said members, and second engagement means within the assembly and engageable with said first engagement means, one of said first and second engagement means including high and low stop means alternately engageable with the other of said engagement means for stopping the axial movement of said axial movable one of said members to effect strokes of different lengths on alternate strokes of said piston means.

8. The pipette assembly according to claim 7 wherein each of said sets of fingers comprises a plurality of equally circumferentially spaced fingers, the fingers of one of said sets being angularly offset from the fingers of the other of said sets.

9. The pipette assembly according to claim 7 wherein each of said fingers is resiliently flexible in a circumferential direction, and said cam means is sized relative to the spacing of said fingers to effect spreading apart movement of adjacent fingers respectively from said first and second sets of fingers during each piston means stroke, and return movement of said adjacent fingers upon each piston means stroke to, hold said rotated member in an indexed position.

10. The pipette assembly according to claim 7 wherein said indexing member is said axially movable member.

11. The pipette according to claim 10 wherein said one engagement means is disposed on said indexing member.

12. The pipette according to claim 10 further including fixed shoulder means connected with said barrel means, and wherein said cam member is engaged by and axially movable with said indexing member into engagement with said shoulder means in response to a predetermined amount of axial movement of said indexing member in a given direction, said indexing member and said cam member being relatively rotatable upon further movement of-said indexing member in said given direction to effect indexing movement between said members.

13. The pipette assembly according to claim 7 wherein said rotated one of said members is said indexing member.

14. The pipette according to claim 13 wherein said indexing member is also said axially movable member.

15. The pipette according to claim 7 wherein said first and second engagement means comprise engaging surfaces spaced from said fingers and lying in planes normal to the axis of rotation of said rotated one of said members.

16. The pipette assembly according to claim 7 wherein the fingers of said first set are in fixed axially spaced relation with the fingers of said second set.

17. The pipette assembly according to claim 16 wherein at least one of said sets of fingers are resiliently flexible and predeterminately spaced from the fingers of the other of said sets of fingers so that said flexible fingers are flexed during strokes of said piston means providing positive rotation of said rotated one of said members.

V UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Damd October 23, 1973 Patent No. 3,766,784

Invemm-( C1arence L. Wa] ker It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the C1a1'ms:

Co1umn'8, Hne 41, "6", second occurrence, shou1d read 5 Signed and sealed this Znd day of April 19%;.

(SEAL) v Attest: 1

EDWARD Mc,FL'JiYICHER,JRo C,a MARSHALL DAMN Commissioner of Patents Attesting Officer U$COMM-DC 6O376-P69 us. eovnmamn PRINTING OFFICE: no! o-su-Ju FORM PQ-105O (i0-69)

Claims

1. A pipette assembly, comprising; barrel means, reciprocal piston means slidable in said barrel means to draw fluid into and expel fluid from a reservoir communicating with said barrel means, pushbutton means for actuating said piston means, means providing alternate changes in stroke length including an indexing member rotatably carried by said piston means, said indexing member having high and low stops thereon, abutment means for selectively engaging said high and low stops on the indexing member, means selectively rotating the stops into alignment with the abutment means including a plurality of axially opposed fingers on said indexing member, cam means engageable with said fingers to rotate the indexing member, said cam means including a first cam surface engageable with one set of opposed fingers and a second cam surface engageable with another set of fingers, said fingers being circumferentially spaced so that when one of the fingers of one set completes engagement with said one cam surface, one of the fingers of the other set is in axial alignment with a portion of the other cam surface so that the indexing member continues rotation in the same direction upon reverse movement of the piston means.

4. A pipette assembly as defined in claim 2, wherein said indexing member is a sleeve surrounding said piston means and rotatable relative thereto, each set of fingers consisting of four fingers cantilevered at each end of the sleeve and projecting toward the other set, one set of fingers being 45* out of phase with the other set of fingers.

5. A pipette assembly as defined in claim 2, said stops on the indexing member including an annular flange on one end of the indexing member, and recessed portions on said flange defining alternate high and low stops.

6. A pipette assembly as defined in claim 5, wherein said abutment means is stationary with respect to the barrel means.

7. A pipette assembly comprising barrel means for communication with a fluid reservoir, piston means axially movable in saId barrel means to draw fluid into and expel fluid from the reservoir respectively on intake and discharge strokes of said piston means, means including pushbutton means for actuating said piston means to move said piston means in one and the opposite direction, and means controlling changes in the stroke length of the piston means and operatively connected with said piston means and said pushbutton means, said means controlling changes in the stroke length comprising an indexing member having connected thereto opposed first and second sets of circumferentially spaced fingers, and a cam member having at least one cam means thereon engageable with said fingers, one of said members being axially movable relative to the other of said members in response to ax1al movement of said piston means in one direction to effect camming engagement between said cam means and at least one of said fingers of said first set, said cam means and said one finger of said first set effecting relative indexing rotation of one of said members in a predetermined direction of rotation by said camming engagement, said axially movable member moving axially in response to axial movement of said piston means in the opposite direction to effect camming engagement between said cam means and one of said fingers of said second set, said cam means and said one finger of said second set effecting relative indexing rotation of said rotated one of said members in said predetermined direction of rotation by said last named camming engagement, first engagement means on said axially movable one of said members, and second engagement means within the assembly and engageable with said first engagement means, one of said first and second engagement means including high and low stop means alternately engageable with the other of said engagement means for stopping the axial movement of said axial movable one of said members to effect strokes of different lengths on alternate strokes of said piston means.

9. The pipette assembly according to claim 7 wherein each of said fingers is resiliently flexible in a circumferential direction, and said cam means is sized relative to the spacing of said fingers to effect spreading apart movement of adjacent fingers respectively from said first and second sets of fingers during each piston means stroke, and return movement of said adjacent fingers upon each piston means stroke to hold said rotated member in an indexed position.

12. The pipette according to claim 10 further including fixed shoulder means connected with said barrel means, and wherein said cam member is engaged by and axially movable with said indexing member into engagement with said shoulder means in response to a predetermined amount of axial movement of said indexing member in a given direction, said indexing member and said cam member being relatively rotatable upon further movement of said indexing member in said given direction to effect indexing movement between said members.